(a) Field of the Invention
The present invention relates to a voltage controlled oscillator. More specifically, the present invention relates to a voltage controlled oscillator with lower power and low phase noise.
(b) Description of the Related Art
A quadrature VCO (voltage controlled oscillator) is a circuit for generating four kinds of signals with the same magnitude but delays of 90 degrees respectively, and is generally applied to direct conversion transmitters and receivers. Direct conversion is a method for converting RF (radio frequency) signals into baseband signals without converting them into IF (intermediate frequency) signals, and is being actively developed since it reduces the number of external components such as filters, and decreases digital signal processing loads. Direct conversion is the most suitable method for a single chip manufacturing process using the CMOS process which easily realizes digital circuits.
FIG. 1 shows a block diagram of a quadrature VCO.
As shown, the quadrature VCO comprises first and second coupled delay cells 110 and 130.
In detail, signals output by (−) and (+) output terminals of the first delay cell 110 are applied to (+) and (−) input terminals of the second delay cell 130, and signals output by (−) and (+) output terminals of the second delay cell 130 are applied to (−) and (+) input terminals of the first delay cell 110.
According to the above-noted configuration, the (−) and (+) output terminals of the first delay cell 110 output signals with the same magnitude and phases of 90° and 270°, and the (+) and (−) output terminals of the second delay cell 130 output signals with the same magnitude and phases of 0° and 180°.
FIG. 2 shows a detailed diagram of a conventional circuit used as the first and second delay cells in the quadrature VCO of FIG. 1.
As shown in FIG. 2, the first and second delay cells 110 and 130 comprise differential VCOs (voltage controlled oscillators) 210 and 230 for varying frequencies of output signals according to a control voltage Vctrl, and fifth to eighth NMOS transistors MN25 to MN28 for coupling the first and second delay cells 110 and 130, the configuration and operation of which will now be described.
The differential VCO 210 of the first delay cell 110 comprises first and second NMOS transistors MN21 and MN22, first and second inductors L21 and L22, and first and second varactors Cv21 and Cv22, and the differential VCO 230 of the second delay cell 130 comprises third and fourth NMOS transistors MN23 and MN24, third and fourth inductors L23 and L24, and third and fourth varactors Cv23 and Cv24.
The first to fourth NMOS transistors MN21 to MN24 generate negative resistance of the differential VCOs 210 and 230, and are cross-coupled.
The first to fourth inductors L21 to L24 and the first to fourth varactors Cv21 to Cv24 form an LC tank, and vary impedance of the LC tank according to the applied control voltage of Vctrl, thereby varying the frequency of output signals.
In the conventional quadrature voltage controlled oscillator shown in FIG. 2, the fifth to eighth NMOS transistors MN25 to MN28 which are coupling transistors are respectively coupled in parallel to a drain and a source of the first to fourth NMOS transistors MN21 to MN24. In detail, drains of the fifth to eighth NMOS transistors MN25 to MN28 are respectively coupled to the drains of the first to fourth NMOS transistors MN21 to MN24, and sources thereof are coupled to the sources of the first to fourth NMOS transistors MN21 to MN24.
Also, the (+) and (−) output signals Q+ and Q− of the second delay cell 130 are applied to gates of the fifth and sixth NMOS transistors MN25 and MN26 of the first delay cell 110, and (−) and (+) output signals I− and I+ of the first delay cell 110 are applied to gates of the seventh and eighth NMOS transistors MN27 and MN28 of the second delay cell 130.
The conventional quadrature VCO shown in FIG. 2 outputs four kinds of signals with the same magnitude but different phases through a relatively easy method, but it has a problem in that low-frequency noise generated by the fifth to eighth NMOS transistors MN25 to MN28 is directly induced to the inductor of the LC tank and the frequency is accordingly transited. This problem deteriorates a phase noise characteristic of the quadrature VCO and increases more phase noise than the phase noise characteristic of the differential VCO.
To solve the problem, another quadrature VCO has been proposed.
FIG. 3 shows a circuit diagram of another conventional quadrature VCO.
As shown in the conventional quadrature VCO, the fifth to eighth NMOS transistors MN35 to MN38 which are the coupling transistors are coupled in series to the first to fourth NMOS transistors MN31 to MN34.
In detail, drains of the fifth to eighth NMOS transistors MN35 to MN38 are coupled to output terminals, and sources thereof are coupled to drains of the first to fourth NMOS transistors MN31 to MN34. The (+) and (−) output signals Q+ and Q− of the second delay cell are respectively applied to the gates of the fifth and sixth NMOS transistors MN35 and MN 36, and (−) and (+) output signals I− and 1+ of the first delay cell are applied to the gates of the seventh and eighth NMOS transistors MN37 and MN38.
The phenomenon that the phase noise characteristic is deteriorated is improved in the above-mentioned quadrature VCO since the low-frequency noise signals generated by the fifth to eighth NMOS transistors MN35 to MN38 are transited to a double frequency of the output signal. However, the quadrature VCO shown in FIG. 3 requires a high supply voltage since the fifth to eighth NMOS transistors MN35 to MN38 are coupled in series to the first to fourth NMOS transistors MN31 to MN34.